Television signals to be processed are, as a rule, assigned to different standards. They differ in the frequency separation and the levels of the picture carrier and the chrominance subcarrier. During transmission, the channel spacing may differ, too. In multistandard equipment or if quasi-split-sound processing is performed, this requires different surface-wave filters (SWFs) in the intermediate-frequency range for channel separation or, in the case of split-sound processing, for separating the video and sound information in the respective channel. If the respective surface-wave filters are not exactly designed for the respective television standard and the respective transmission standard, and the selectivity skirt in the range of the adjacent picture carrier, the relatively high picture-carrier amplitude of the adjacent channel may show up as an interference signal both in the two sound carrier channels of the current television channel and in the picture. If the interfering picture carrier amplitude is large enough, it enters as a disturbance variable into the large-signal behavior of subsequent stages. In that case, the unavoidable non-linearity of a subsequent analog-to-digital converter is no longer negligible for the signal processing. Furthermore, the resolution of the analog-to-digital converter decreases since an essential portion of the available dynamic range is occupied by the signal amplitude of the adjacent picture carrier, whereby the number of quantization levels available for the desired signal is reduced.
Surface-wave filters, which, as a rule, cannot be fabricated together with monolithic integrated circuits, represent separate components which increase the amount of external circuitry and the components count, particularly in multistandard equipment.
It is an object of the invention to reduce the additional external circuit complexity caused by the use of surface-wave filters, particularly in multistandard equipment, to a minimum.